Since aluminium-zinc hot-plated steel plate has the features of good corrosion resistance, high heat reflectivity, heat resistance and ability for appearance decoration, it is widely used in various fields of the people's life, such as buildings, household appliances, automobiles etc., for example, a roof and wall surface of lightweight steel construction, a purlin, a floor support plate, a guardrail, a sound barrier and a drainage etc. in the building industry, and a back board of large-scale electric appliances and an automobile exhaust system etc. in the household appliance field. When particularly used as a building material, because of the need of guaranteeing a service life of more than ten years or even several decades, relatively strict requirements of the aluminium-zinc hot-plated steel plate are raised: for example, the material for a roof and wall surface needs to have good weather resistance and corrosion resistance, so as to ensure that the appearance surface of the steel plate during service has no colour change and corrosion, and the beautiful appearance is maintained for a long time; and when the material is used as a floor support plate, a guardrail, a sound barrier and a drainage etc., it is often in contact with slating and concrete etc. and thus exposed to an alkaline corrosion environment, in which case the material must be required to have good alkali resistance etc.
In order to meet the above-mentioned requirements and expectations, the surface of the aluminium-zinc hot-plated steel plate would usually be subjected to a wet film-coating treatment to enhance the resistance of the aluminium-zinc hot-plated steel plate to various corrosive media and environment, and currently the types of the surface treating agents used for the aluminium-zinc hot-plated steel plate can be roughly divided into two categories of chromium-containing treatment and chromium-free treatment. For the building industry, the corresponding environmental regulations have not been issued yet, and chromate has extremely strong corrosion resistance and self-repairing function, such that the aluminium-zinc hot-plated steel plate treated with the chromium-containing surface treatment becomes the mainstream in the building industry nowadays; however, with the development of the society, environmental protection requirements are increasingly strict, and the environmental protection of the building industry has been an irresistible trend; therefore, the aluminium-zinc hot-plated steel plate treated with the chromium-free surface treatment having various excellent resistances will be the mainstream in the future.
Japanese Patent JP 2007-321224 discloses an aluminium-zinc hot-plated steel plate having excellent corrosion resistance and water-resistant adherence, wherein the surface thereof is covered with a chromium-free treated organic/inorganic composite protection film, and the protection film consists of a silane-modified aqueous anionic resin, a tetravalent vanadium compound, and phosphoric acid or a compound thereof etc. The aluminium-zinc hot-plated steel plate coated with the protection film has excellent corrosion resistance and water-resistant adherence, but does not have adequate weather resistance and alkali resistance.
Chinese Patent CN 1530462 A provides a treating agent for metal surface and an aluminium-zinc plated steel plate treated with the treating agent, wherein the surface treating agent comprises an aqueous resin with combined carboxyl group and acid amide, such as a urethane resin or an acrylic resin, an organic silane coupling agent and one or two metal salt compounds, and the above-mentioned treating agent is coated onto the surface of the steel plate to form a single layer of skin film of 0.2-5 g/m2. Although this patent provides a chromium-free coating film having good corrosion resistance and alkali resistance, the weather resistance thereof is not mentioned.
Chinese Patent CN 1511908 A provides an aqueous resin composition for treating a steel plate plated with an aluminium-zinc alloy, which comprises an anionic water-dispersed polyurethane resin, a silane coupling agent and a water-soluble zirconium compound etc., wherein the steel plate having an aluminium-zinc alloy surface to be treated with the treating agent has high processability, high corrosion resistance, weather resistance, resistance to blackening, and adhesion to a coating film, and this treating agent is also an environmental-friendly product; however, the evaluation time in this patent for the weather resistance is only 100 h, which is relatively short.
Chinese Patent CN 124777 A discloses an aluminium-zinc alloy steel plate coated with a resin composition, wherein a silane coupling agent having an amino group, chromium ions and an alcohol at least selected from one of a trihydric alcohol and a dihydric alcohol with 2-3 carbons are incorporated into an acrylic polymer resin emulsion containing a carboxyl group and a glycidyl group and having an acid value of 10-60, such that the resultant mixture has a pH of 7-9 so as to obtain the resin composition; and the resin composition is coated onto the aluminium-zinc alloy steel plate, and dried to form a resin skin film. The aluminium-zinc alloy steel plate coated with the resin composition has good formability, corrosion resistance, alkali resistance and coatability. However, the designed treating reagent therein is a chromium-containing product, and does not have an environmental protection performance, and the weather resistance thereof is not investigated.